Molded case circuit breakers are commonly used in multi-metering panelboards for commercial and industrial applications that require hundreds, sometimes even thousands, of amperes of current to pass through the circuit breakers. In a short circuit condition, these current levels produce significant magnetic forces on the conductors. Substantially rigid busbars are typically used to connect the load side of the breaker to the load lines, but the line side of the breaker is typically equipped with lugs that can be connected to flexible cables leading to the power distribution lines. The magnetic forces produced by a short circuit is condition can cause whipping of the cables, which exerts significant forces on the lugs and the adjacent portions of the molded case. These forces can pull the cables out of the lugs, displace the lugs, and even cause cracking of the molded case. The cable whipping tends to become more severe as the length of the unsecured cables increases.
One way to reduce the movement of the cables during short circuit events is to “brace” the cables by tying all of the cables together with rope. This practice of “bracing” the cables is common on switchboards with high short circuit current ratings. (See UL 891 paragraphs 6.3.3, 9.2.4.2.4.1, G5.1 and Figure G5.1.) Bracing is primarily intended to prevent the cables from coming out of the lugs, but also reduces movement of the lugs and thus reduces the risk of fracturing the molded case of the circuit breaker. However, the use of cable “bracing” is not always appealing to the customers responsible for implementing it. It is also possible to design reinforcements into the molded case of the circuit breaker, but this increases the cost of the breaker, for a problem that is encountered in only a small percentage of the applications for such breakers. Thus, there is an ongoing need for improved techniques for protecting molded case circuit breakers from the effects of short circuit conditions.